A common sculpting medium is clay, which is of mineral origin and comes either in water-based or oil-based form. Water-based clays are typically used by the artist or craftsperson to fashion pottery or small figurines. Drying of such objects is a tedious process so that the result is free of cracks. Firing is required to insure permanency. Water-based clays generally do not come in a range of colors except those which are the natural coloration of the mineral filler, typically gray or rust brown. Coloration must be applied to the surface after firing and it too must generally undergo a second firing to preserve the color. After completion, the article is heavy, fragile, and brittle. Oil-based clays are produced in a range of malleabilities and often come in a range of colors. These products, however, are non-drying and incorporate pigments which can stain textiles, carpeting, furniture, and other surfaces.
Another class of modeling compound is of vegetable origin. Some incorporate starch as the filler and are primarily intended for children. Such doughs have low viscoelasticity, are easily malleable, and come in various colors. Because it is the binder that is pigmented, color brightness is generally lacking. Further, surfaces, particularly carpets and other textiles, are easily stained. The dough, once it is in a carpet, is very difficult to remove. Starch based doughs are unsuitable for fashioning permanent sculptures as drying induces considerable distortion and cracking of the finished object. Cellulose fiber is another material of vegetable origin which has been used as a filler in modeling doughs. Often blended with other mineral fillers, these doughs are reasonably suited for dried sculptures, albeit with some shrinkage and distortion occurring while the sculpted object is drying. These compounds are generally available only in gray or white. Use of this material is difficult since it is very tacky (sticky) and lacks cohesion in the wet state.
Another class of sculpting medium is polymer based. One such formulation incorporates polyvinyl chloride as the filler. Formation of a permanent sculpture usually requires a curing process at elevated temperature, which causes the material to release toxic gases. The material itself is somewhat toxic and is not suitable for use by children without adult supervision. Another product formulation is composed essentially of hollow polyvinylidene chloride microspheres combined within a gelled polyvinyl alcohol binder. It is lightweight and dries with relatively little shrinkage, and due to its viscoelastic properties, it is resilient and bounceable. A viscoelastic material combines both fluid and elastic properties, but viscoelasticity compromises its utility as a sculpting medium. Having an elastic component, the material is difficult to shape in the precise detail intended by the sculptor. Increasing the water content reduces this property but dramatically increases its tackiness, i.e., its tendency to stick to the hands and other surfaces. Coloration is normally accomplished with pigments which are blended into the binder, but these pigments can cause staining on textiles and other surfaces coming into contact with the material.
U.S. Pat. No. 5,157,063 discloses polystyrene foam beads, 1-2 mm in diameter as a filler and a mixture of polyvinyl alcohol, polyvinyl acetate glycerin and xanthan gum as the binder. The use of gelled polyvinyl alcohol/acetate produces a viscoelastic material and therefore tends to rebound. The rebounding attributes contribute a negative effect on the product as the material tends to return to its initial position.
U.S. Pat. No. 5,171,766 discloses a moldable composition comprising polyvinyl alcohol, water, a gellant and a filler of plastic microspheres having a wettable particulate coating. The preferred gellant is sodium tetraborate. The gelled polyvinyl alcohol base produces a dough which is viscoelastic. A wettable particulate coating is required to insure that the filler is retained with the binder.